Modelling of Dielectrically Loaded Bifilar and Quadrifilar Antennas

TitleModelling of Dielectrically Loaded Bifilar and Quadrifilar Antennas
Publication TypeConference Paper
Year of Publication2000
AuthorsLeisten, O, Agboraw, E, Vardaxoglou, JC, McEvoy, P, Cai, M, Wingfield, A
Conference NameProgress In Electromagnetics Research Symposium 2000
Conference Start Date05/07/2000
Conference LocationBoston, MA, USA

The paper discusses time-domain modellingof dielectric loaded bifilar and quadrifilar helix antenna topologies using TLM and FDTD. This antenna has been the subject of considerable research usingelectromag netic modellingand empirical techniques. Results from a study of miniaturised quadrifilar helix antennas employingceramic dielectric materials to dielectrically foreshorten the resonant dimensions have been published. From a computational electro-magnetics point of view this has proven to be a complex structure to simulate. the paper will demonstrate how measures to simplify the model to achieve computational efficiency have significantly reduced the predictive accuracy of the model. More realistic models that are able to advance the design is described with a critical appraisal of the predictive accuracy. The approach to castinga realistic model that is able to advance the design is discussed with a critical appraisal of the predictive accuracy. The proposed antenna falls into the category of small antennas and the simulations account for the high field concentration in the dielectric as well as the intricacies in the input impedance and feed mechanisms. The antenna is formed as a cylindrical ceramic dielectric core. It offers advantages over conventional antennas with regard to the isolation of the currents on the antenna element from those on the hand-set ground-plane and near field detuning. It is targeted for use in GPS instruments that are small enough to be used in cell phone handsets and also in Personal Communications Services. The main modellingto ol that was used to model the crown-rimmed antenna was a commercial TLM electromagnetic simulator called MicroStripesTM. The design methodology was to estimate dimensions by analytical means based on measured line parameters and to submit this antenna to a manual optimisationregime based on a battery of field solvers. Although MicroStripesTM can be supplied with programs which provide a means for graphical entry it was found to be more efficient to use custom Visual BasicTM programs to transcribe critical dimensions into the build language script that is interpreted by MicroStripe’s geometry builder. In this way it was straightforward to submit a large number of optimisation trials to the simulator very quickly. The FDTD program uses Perfectly Matched Layers (PMLs) as absorption boundary condition to confine the computation domain. However, both methods require huge amount of memory to achieve the higher resolution of the modellingob ject under simulation. Currently, we have used over 300 Mb of memory for a spatial resolution of 0.3 mm. The radiation pattern for circular polarisation and input impedance will be discussed.

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